Glass-to-metal seal



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' "form a satisfactory seal between gl'ass and metal Patented July 25,1939' UNITED STATES.

PATENT OFFICE I Gmss-tro-Mn'mn SEAL,

Albert W. Hull and Emmett E. Burger, Schenectady, N. Y., assignors toGeneral Electric Com- P a corporation of New York Application May 15,936, Serial No. 19,950

Renewed A m 15, 1939 scram. 49-81)- The present invention relates toii'nproved' strain-free glass-to-metalseals andto combinations ofmaterials suitable for producing the same. i It has previously beenknown that inorder to the stresses. resulting from thermal expansion andcontraction during the sealing period must not exceed the tensilestrength of the glass. Until quite recently it hadbeen assumed that thiscondition would be suiiiciently fulfilled if substantial equalityexisted between the average 00-.

efficient of expansion of the metaland that .of the glass as measured byobserving the expansion of the latter at a temperature belowtheannealing point. I

It is pointed out in an article by Dr. A; W. Hull and Mr. E. E. Burgerappearing in vol. 5 No. 12, of Physics for December, 1934, that thethermal expansion characteristics of the various glasses are not linearfor all temperatures but, on the contrary, possessja transformation zoneof rapidly changing expansibihtyfthis zone covering a range oftemperatures appreciablybelow the annealing point. The authors furthershow that glass-to-metal seals will be strain-free at all temperaturesif, and only if, the coefliclent of expansion of the metal varies fromroom temperature to approximately the softening temperature of the glassto which it is to be sealed along-a thermal characteristic curve whichessentially matches that of the glass at all points both above and belowthe transformation zone of the lat-' ter. The maximum divergence whichcan be 'tolerated at any point between the thermal characteristics ofthe glass and metal is determined by the condition that strains setup-duringcooling must be less than the -strength of the glass.

As an example of a material which adequately fulfills the requirementsfor productionof' a strain-'free'seal the authors describe an alloy of'iron, nickel and cobalt (54% Fe,,28% Ni and 18% C0) which has a thermalexpan ion curve very similar to that of a standard hard glass (Corning'6-71). While seals comprising these materials have proven to bemechanically-very satisfactory,

they possess certain disadvantages-from a com-. mercial point of view inthat the cobalt alloyrequired is expensive and thatthe glass used isworkable only at relatively high temperatures.

It isan object of the present invention to produceanewglass-tovmetalseal in which the glass and metal have thermalcharacteristics which [substantially correspond in value and slope. at

temperatures both above and below the trans-'- formation zone .oftheglass and which utilizes a duceysuch a seal employing a'glass whosesoftenclass'of relatively inexpensive nickel-iron alloys.

Itis a further object of the invention toproing point is low enough topermit the utilisation in hydrogen at 500 C.

on the vertical axis: For curve group II subtract 7 of'cheap fuels suchas ordinary city gas and air in working .the glass, and which at thesame time suffers no chemical reduction when it is heated It is afurther object ofthe invention to utilize l in an improved seal aglasshaving the above properties which meets the additional require;

ments of flowing readily when-molten, of wetting the metal to which itmust be bonded, and

of having a relatively high electrical resistivity.

These objects are attained inuaccordance with the present invention byusing a nickel-iron alloy comprising from about 39.to about 44 per centnickel in" combination with a bore-silicate glass of a class more fullyspecified hereinafter having an ll alkali content of from about 2 to '7per cent.

The novel features which we believe to be characteristic of ourinvention are set forth with particularity, in the appended claims. Ourinven-'- tion itself, together with further. vobjects and ad- 20vantages thereof, will best be understood by reference to the followingspecification taken in' connection with the accompanying drawing inwhich Fig. 1 comprises a series of curves illustra-ting the thermalcharacteristics of the compopents of our improved seal; and'Flgs. 2 and3 illustrate the application of the invention to .particular structures;In order to prevent a confusing overlap of the various curves-of Fig. 1,

each curve group with the exception of group I h has been verticallydisplaced from its normal position. For this reason the followingprecautions must be observed in reading contraction values h as .oo1

from the apparently indicated value; for group III subtract t,

' cm v I .0023; and for group IV subtract The thermal characteristics ofnickel-iron 5.1- loys containing from 25*per cent to 60 per centnickeldifi'er striklns yifrom those of most other metals in exhibitingdistinct transformation zones in which the c'oeflicient of expansion (orcontraction) abruptly changes with increase in tempera tures. Inparticular, nickel-iron alloys havinga nickel content in the'range fromabout 39 to 43 per cent, which range we consider to be com-. prisedwithin the scope-of our invention-have itransformation zones below about425 degrees C. and ranging from that value :down to about While thetransformation zones mentioned in the foregoing are of similar nature tothose which so Table A Per cent S102 52 to 64 B203 29 to 40 NazO 2 to 7A1203..."- 7 0 t0 5 characterize most glasses, it-has heretofore beenconsidered impractical to produce a satisfactory seal between thesealloys and the glasses previously known due to the fact" that thetransformation zones of the latter in most cases occur considerablyabove 400 C. In other cases the average coefficients of expansion of theglasses differ so widely from those of the alloys that a sealing matchis entirely out of the question.

In accordance with our present invention we have found that verysatisfactory strain-free seals may be made by combining nickel-ironalloys containing from 61 to 57 per cent iron and a complementaryproportion of pure nickel or nickel modified by slight percentages ofcobalt with a boro-silicate glass of the type described and claimed in aco-pending application, Serial No. 79,877, Louis Navias, filed May 15,1936. We have further found that by properly adjusting the compositionsof the metal and glass a large number of combinations can be determinedwithin the specified range such that the thermal characteristics of thetwo materials correspond to the extent necessary to producesubstantially strain-free seals.

Certain glasses which have been found to be suitable for the purposes ofthe invention fall within the following range of compositions:

. oration.

A distinctive aspect of the glasses which aresuitable for the practiceof our invention consists in the fact that they comprisea boro-silicatecomposition containing a percentage of alkali material (such as sodiumorpotassium oxide). in a range of from about 2 to about 7 per cent. Due totheir low softening points, which are uniformly less than about 675 C.they are able to be worked very readily in a flame using an inexpensivefuel such as a mixture of ordinary city fuel gas and air. Furthermore,they are uniformly distinguished by, thermal expansion characteristicswhich have transformation zones 00- curring below about 425 C. and inmost cases below 400 C.

Referring now to Fig. 1 of the drawing we have shown the thermalcontraction characteristics of typical glasses and alloys coming withinthe specified range and combined in accordance with the principles ofour invention. By the term "con-g 4 acteristic which measured thespecificincrease in length for a one degree rise in temperature. It is,however, regarded as a somewhat more reliable index of the properties ofthe glass in bei g entirely independent of the previous thermal historyof the specimen of glass selected for testing.

In considering Fig. 1, it will be noted that the contraction curve(characteristic) of ,each glass corresponds substantially to that of itsassociated metal, and that the transformation zone of each materialoccurs at about the same temperature range.. In view of the resultingabsence of any considerable divergence between the contractioncoeflicients of the selected pairs of materials, any seal in which theyare combined will be essentially strain-free not only at the annealingtemperature but also at all other temperatures below that value.

It should be noted that if the-characteristic curves of the glass andmetal are of generally similar slope at temperatures above and below thetransformation zone, a slight divergence between them will not result.in serious strain accumulations, even where the mass of the metal islarge in comparison with that of the glass.

The composition of the particular alloy associated with each of. thecurves of Fig. 1 is indicated by the legend attached thereto while thecomposition of the corresponding glass may be obtained from thefollowing table in which the index letters-correspond to those of thedrawing. As explained in the aforementioned Navias application, SerialNo. 79,877, the sodium oxide components may be replaced by equivalentproportions of other alkaline oxides such as potassium or lithium,taking into consideration the known differences in these materials.

Table B A B O D SiOg 54 64 60 58 B10- 40 30 31 33 Nag O 3 i 6 6 A110: 33 3 Among the various examples given in the drawing we considercombination H, employing a 41 per cent nickel-iron alloy and the glassdesig: nated as "Glass C", to be a, preferred embodiment of ourinvention. The glass of this combination we have found to possesssomewhat greater stability than those of lower alkali content and, atthe same time, to' possess a transformation zone sufficiently low toenable it to be matched by an alloy whose low nickel contentconsiderably decreases its cost.

It should be pointed out that the glass materials which employ an amountof' boron oxide '(BzOa) in excess of about 35 per cent tend to suffersome deterioration when exposed to moisture under adverse circumstances;For this reason we consider it desirable in the use of seals embodyingsuch glasses to provide the same with a coating of a material capable ofexcluding moisture such for example as a baking enamel of, the alkydresin type.

The glass alloy combinations labeled on the drawing as II and IIIrespectively are illustrative of the metal combinations consisting ofiron and commercially pure nickel. It is recognized, however, thatcobalt is a common impurity in most available supplies of nickel, and wehave found that its presence as a modifying element may prove usefulrather than otherwise in helping to control the transformation zone ofthe alloy. Thus, by the addition of small percentages of cobalt up to abut 5 or 6 per cent the transformation zone of a particular alloy may beraised "to correspond more closely with that oi a particular glass inthe speciiied group. We there fore consideras-included within the scopeof our invention seals embodying alloys of iron with a nickel-cobaltcombination in the sum of from 39 to 44 per cent and in which the cobaltmeasures from zero toabout or 6 per cent.

The group of curves designated by the numeral I show the contractioncharacteristics of a pre-- ferred glass and of a pair of closely matchediron alloys containing nickel and cobalt. It will be other. It'should beunnry to point out that slight variations in the glass composition becompensated for by corresponding changes in the all'oy percentageswithout departing from the scope oi our invention.

In addition to cobalt very small quantities of manganese and silicon maybe considered desirable constituents of the alloy from the point of viewof improving its workability. However. be-

yond about 0.5 per cent either of these ingredients may be sumcient tochange the expansion characteristic of the alloy to an extent which.

interferes with good sealing. An alloy composition which approaches thislast-named limit and a glass which aflords a fairly good match therewithis shown in curve combination IVs Various applications of the inventionare illustrated in Figs. 2 and 3 of the drawing. "'Inj 'ig. 2,

'for example, we have shown a flared stem iadaptedtobeseaiedintothebaseoiaglassenvelope. The stem terminates at oneend in a press 2 into which is fused a pair of lead-in conductors 3,These conductors may consist of of the alloys described in.theforegoing,-'while the stem, or at least the pressed portion thereof,should consist of a glass of the class pe ified havingthermaicharacteristics matched with those of the lead-in conductors. y

In constructing a glass-to-metal-seal pi this type the glass is, firstheated sufiiciently to enable it to wet a metal surface. Forthe.particinar glasses described this will occur at a temperaturewhichjis unusually low in comparison with other known glassesof the lowexpansion class. Thereafter the conductors 4 may be inserted and theglass pressed'down and cooled at a uniform rate to room temperature. Dueto the similarity of the contraction characteristicof the glass andmetal upto the softening temperature of the former, a strain-iree sealmay be obtained withv out the necessity of holding the combination for arelatively long time at the so-called annealing temperature (defined ias the temperatures at which9qpercentofthestrainsintheglasswill'.be.eliminatedinaperlod of about 15 minutes).

The only limitation asto the rate of'cooling that need be observed i'sthat which will avoid internal thesealduetounequalcooling'.

In Fig.3 we have shown an alternative application of the invention in onwith a socalled "mitt-seal". In this embodiment a pair or strains in theportion-of the glass-remote from metal s of anickel-iron within thescope of our ihventionare ioinedgby direct fusion to an intermediatecylinder lof, a glass of the type specified herein having matchingcharacteristics. Due to the fact that the combinations which we havedescribed are essentially strain-free at all temperatures withintherange covered during the sealing operation, limitations of seal sizeare practically non-existent insofar as these depend on diflerences inthermal characteristics. Thus. seals of the form illustrated may be madeutilising cylinders as largeas six inches or more in diameter and of anydesired wall thickness. Even this size is not limiting provided dueprecautions'are observedwith respect to obtaining equal rates of heatingand cooling for the various metal and glass parts.

What we claim as. new and desire to secure by.

Letters Patent of. the United States 13:.-

1. A glass-to-meta'l seal comprising a glass which contains from about50 to 64 percent 810:. from about 23 to 40 per cent 810:, and from about2 to 7 per cent of an'alkali material in combination with an alloyconsisting essentially of from about 39 to 43 percent nickel withacomglementary proportion of iron, said glass and said alloy havingthermal contraction characteristics which subs both above and below thetransformation zone.

2. A strain-free glass-to-metal seal comprising a glass which containsabout 60 per cent SiOs,

I about 31 per cent 310:, about 6 per cent NasO,

and a complementary proportion of M10: in combination with an alloyconsisting essentially of about 41 per cent nickel and 59 per cent iron.

3. A glass-to-metal seal comprising a glass which contains about 58 percent 510:; about 33 per cent 1310:; about 6 per cent NaaO; about 3 percent in combination with an alloy contemperatures both aboveand belowthe transformation zone.

4. A glass-to-metal seal Q mDrising a glass the oxide composition ofwhich contains about 58 per cent 810:, about 33 per cent 810:, about 6per cent NaaO, and about 3 per cent shot in combination with analloyconsi'sting essentially of iron combined with nickeland cobalt in asum tantially correspond at temperatures sisting essentially'of ironcombined with nickel.

of about 42 to is per cent, and in which the cobalt comprises from I to5 per cent, said glass andsaid alloy having thermal. characteristicswhich sub,-

stantially correspond at temperatures at, above and below thetransformation zone of the glass.

5. A glass-to-metal seal comprising a glass' which contains from aboutto 64 per cent SiOs. from aboutfltowpercentvBsOaandfrom aboutlto'lperbentof analkalimateriali'n combination with an alloy consisting.essentially of iron combined with nickel and cobalt in a sum of fromabout 39.to44 per centand infwhich the cobaltcomprises from o to 5 percent. said glass and said alloy having thermal contraction'cha'racteristics which substantially correspond at temperatures both aboveand below the transforms tionsone. ALBIBT W..HUIL.

4 MITIIBUBBIR.

